The present invention relates to using microwaves for measuring density and/or moisture content of materials, and more particularly, to a method, and corresponding device and system thereof, using surface microwaves for measuring and determining density and/or moisture content of a material. Microwaves are transmitted into a surface microwave waveguide having an externally exposed flat top surface immediately above which spans a proximate free space region within which the material is positioned. A portion of the transmitted microwaves propagate within and through the proximate free space region including the material, such that parameters (amplitude, phase, attenuation, and phase shift) of the propagating microwaves are perturbed by the material and are a function of density and/or moisture content of the material positioned within the proximate free space region. The propagating microwaves exiting the surface microwave waveguide are received, and the parameters (amplitude, phase, attenuation, and phase shift) of the transmitted and received microwaves are useable for determining the density and/or moisture content of the material. The present invention is generally applicable for measuring and determining density and/or moisture content of a variety of different types of dielectric materials, loose or packed, and particularly applicable to in-line measuring and determining density and/or moisture content of a relatively low density thin sheet, board, or web, of dielectric material which is continuously or intermittently moving along its longitudinal axis during the surface microwave material inspection procedure.
Manufacturing of a wide variety of consumer products, in a wide variety of industries, involves in-line processing of materials. This includes in-line processing of materials at an early, intermediate, advanced, or final stage in the overall commercial manufacturing sequence leading to a finished product. The present invention is particularly applicable to that stage of an overall commercial manufacturing sequence involving continuously or intermittently transporting or conveying a material between manufacturing processes. There exist overall commercial manufacturing sequences including a stage whereby raw or initially processed material exiting an upstream manufacturing process is formed into an intermediate material, which is then continuously or intermittently transported or conveyed to further downstream processes, eventually leading to production of bulk quantities of finished consumer product.
In going from one process to another process in a given manufacturing sequence, bulk density, moisture content, and consequently, uniformity, throughout the length of the transported or conveyed material may change, according to the particular operating or processing parameters of the processes involved. At this stage of the manufacturing sequence, bulk density, moisture content, and uniformity, are often important properties of the material which need to be determined, monitored, and controlled, prior to the material entering further downstream processes or storage. In particular, if the bulk density and/or moisture content of a given material are outside of established quality control values, use of such material is expected to lead to downstream intermediate, advanced, or final stage materials, or stored material, similarly failing their established quality control values, potentially causing undesirable rejection of material, manufacturing down time, and added cost to the finished product.
Microwave techniques, known from the prior art, are available for non-invasively determining bulk density and/or moisture content of a variety of different types of dielectric materials, during in-line processing. In these teachings, typically, a microwave radiation source beam is transmitted by a first antenna through a portion of material and the transmitted microwaves are received by a second antenna, which then produces a signal characteristic of the material and properties therein. Microwave signal parameters such as amplitude, phase, attenuation, and phase shift, are used to determine bulk density and/or moisture content of the material. Such techniques are applicable to relatively high bulk density loose or packaged materials, for example, having bulk densities of on the order of several kilograms per square meter, and provide measuring sensitivities significantly less than desirable for many current manufacturing sequences involving in-line processing of thin sheet, board, or web, configured materials. In particular, cost and quality of many current manufacturing sequences involving in-line processing of relatively low density thin sheet, board, or web, configured materials could be significantly improved if there was to exist the capability of determining and monitoring bulk density in the range of from about one kilogram to essentially zero kilogram, with corresponding measuring sensitivity of the order of one gram per square meter.
Microwave techniques for measuring density and/or moisture content of a given material are based on the direct relation of these material properties and characteristics to the dielectric properties of the material. There exist a wide variety of microwave devices which operate on the principle of transmission of microwaves through dielectric materials. These devices find wide application in laboratory measurements as well as in production lines, and they provide rapid and highly accurate measurement results.
In microwave moisture measurement, a prominent role is played by microwave devices having two antennas—one for emitting microwaves towards the dielectric material being tested, and the other for receiving the microwaves which pass through the material. The directional patterns of these antennas are such that most of the energy of the microwaves passes through the probed material. This condition determines the manner of construction and geometrical configuration of the antennas of microwave devices.
Antenna based microwave techniques find application in the measurement of density and moisture content of objects having cross sectional dimensions comparable to, or in excess of, the operable dimensions of the antennas, and whereby the path length of the microwaves within the material amounts to several wavelengths. Several examples of such antenna based microwave techniques for determining density and/or moisture content of materials are described in disclosures by the same applicant of the present invention: U.S. Pat. No. 6,107,809, entitled: “Device And Method For Non-invasively Determining Moisture Content And Density Of Loose And Packaged Tobacco”; U.S. Pat. No. 6,111,415, entitled: “Device And Method For Determining The Moisture Content Of Bulk Material”; U.S. Pat. No. 6,025,724, entitled: “Device And Method For Determining The Moisture Content Of Packaged Material”; U.S. Pat. No. 5,845,529, entitled: “Device And Method For Determining The Moisture Content Of Material”; and U.S. Pat. No. 5,621,330, entitled: “Device And Method For Determining Moisture Content Of A Bail Of Material With Multiple Layers”.
In cases where the path length traveled by the microwaves within the material is less than one wavelength, sensitivity and resolving power are no longer sufficient, and to measure, for example, a thin sheet, board, or web, of dielectric material, methods and devices have been used involving multiple passage of microwaves through the probed material. Such methods and devices are disclosed in U.S. Pat. No. 4,620,146, and in U.S. Pat. No. 5,886,534. A significant limitation of these methods and devices is the occurrence of undesirable high energy losses. Lost energy inevitably finds its way into side lobes of the antenna directional pattern. Sensitivity, accuracy, and precision, of measurements are also limited by multiple diffraction and wave interference.
In the case of measuring density and/or moisture content of a relatively low density long continuous sheet, board, or web, of material, high sensitivity, accuracy, and precision, are achieved by using microwave waveguide devices. A well known method of measuring moisture content of a continuously moving thin sheet, or, woven or web, configured material, is based on using a hollow curved waveguide, for example, as disclosed in U.S. Pat. No. 3,628,135. The disclosed method suffers from the drawback of using a microwave device having relatively complex structure within which the waveguide must be configured along the direction of propagation of the microwaves while the material must be led in through a narrow slit. If this slit is widened, then energy losses inevitably increase due to sideways emission of the microwaves, and additionally, it is difficult to use such a device for measuring moisture content of loose materials.
The invention disclosed in PCT Publication No. WO 97/21992 describes a method and apparatus for measuring moisture content of materials which are based on a closed hollow waveguide with a series of slits cut into one of its walls. Part of the energy of the microwaves propagating within the waveguide is outwardly emitted through the slits. The moisture content of the material positioned close to the slit wall is measured from the coefficient of reflection of the microwaves propagating within the waveguide, which permits the determination of the moisture content of the material. The required sensitivity may be achieved for dense and moist materials which have a high coefficient of reflection, such as damp, freshly sawed, wooden planks. A thin sheet, board, or web, configured material, as well as low density loose materials, such as tobacco, are difficult to measure by this method since the coefficient of reflection from the surfaces of these materials is relatively low.
There is thus a need for, and it would be highly advantageous to have a method, and corresponding device and system thereof, using surface microwaves for measuring and determining density and/or moisture content of a material. Moreover, there is a need for such an invention which achieves high sensitivity, accuracy, and precision, of the surface microwave material inspection procedure. Additionally, there is a need for such an invention which is generally applicable for measuring and determining density and/or moisture content of a variety of different types of dielectric materials, loose or packed, and particularly applicable to in-line measuring and determining density and/or moisture content of a relatively low density thin sheet, board, or web, of dielectric material which is continuously or intermittently moving along its longitudinal axis during the surface microwave material inspection procedure. Additionally, there is a need for such an invention which is relatively inexpensive to construct and implement, and which is especially suitable for application in commercial manufacturing environments.